Not-so-resonant, resonant absorption

Abstract

When an intense electromagnetic wave is incident obliquely on a sharply bounded overdense plasma, strong energy absorption can be accounted for by the electrons that are dragged into the vacuum and sent back into the plasma with velocities v=v, . This mechanism is more efficient than usual resonant absorption for v,+co) L, with L being the density gradient length. In the very high-intensity CO2-laser-target interaction, this mechanism may account for most of the energy absorption. PACS numbers: 52.50.Jm, 52.25.Wz, 52.75.Di When a very intense laser field is incident obliquely on a metallic surface or a sharply bounded overdense plasma, a large absorption rate can be accounted for by electrons that are pulled into the vacuum and sent back into the plasma with v=v. ""with v",=eE/m, co being the quiver velocity. This mechanism is eA'ective when the overdense plasma density is well over the critical density and when a strong density gradient or discontinuity is present; the presence of an underdense plasma is not necessary. Those conditions are met in the laser-grating accelerator concept, ' where an intense laser field incident on a grating is used to accelerate electrons. For this purpose, high-power CO2 lasers are now available that can deliver up to 100 mJ in a time scale of 1 ps: A short enough time scale is necessary if we want to maximize the electric field without damaging the grating surface. I find that the present mechanism has a strong limiting eAect on the magnitude that the electric field normal to a grating can reach without significant absorption of the wave energy. In the laser inertial fusion context in the very-high intensity regime (i.e. , Ik~) 10'6 W pm /cm ), a strong density steepening with an overdense region where the density may be as large as 40no (no being the critical density) arises in the region around critical density.Under those conditions, the present mecha-E(xt =0) =E, ", (tt)+AEt =0 or AEt = -E,",(tt), nism may become more efficient than resonant absorption to absorb energy. Also, some laser fusion schemes involving a magnetic field may become more attractive since, with this new mechanism, the energy absorption takes place directly at the focal spot (the dragged electron goes back to the overdense plasma in about one cycle).To describe the energy absorption mechanism, let us consider a one-dimensional capacitor model where we have at x~0 a perfect conductor which can freely emit electrons. For x (0 I assume a vacuum region where a uniform electric field E "t = E p sin cot is present in the x direction. As the field builds up for t )0, electrons will be pulled out in order to maintain a zero field on the conductor surface at x =0. The lth particle that is emitted at time t =tI will see a field E(xi) =E, ",+hE(, where t Xl(t) AEt = -4treJ I & ndx(2) comes from Poisson's equation with n being the electron density and x 1 (t) and xt(t) are the positions of the first (left-most) particle and lth particle, respectively. Since the particles cannot overtake one another, AFI is constant...

Attosecond control of collective electron motion in plasmas

“…1b). During this process, electrons carry away the energy they acquired in the light field: this is the so-called Brunel absorption [18,19]. The combined effect of both the laser and space charge fields is to make faster electrons return to the plasma surface at later times during the laser cycle.…”

“…To demonstrate this, we developed a simple model, inspired from the one developed in [18] and similar to the one described in [12], which gives direct insight into the measurements of emitted by the background plasma oscillations. Because in CWE, the shape of the attosecond pulse is almost independent of the amplitude of the light wave, the time structure of the APT is almost exclusively determined by the relative timing of XUV emission from one wave cycle to the next (Fig.…”

Investigations of Field Dynamics in Laser Plasmas with Proton Imaging

“…The angular dependence of the Brunel absorption can be estimated analytically assuming a step-like density profile and a relativistic increase of the electron mass. More details can be found in references [17,51]. As a result of the analytical discussion two limiting cases will be presented, for a 0 ≪ 1 and a 0 ≫ 1.…”

“…This mechanism is named after F. Brunel who published the famous article "Not-so-resonant, Resonant Absorption" [51] in the year 1987. If the oscillating electrons along the density gradient with the amplitude x p ≃ eE L /m e ω 2 L = v os /ω L exceed the density scale length v os /ω L > L the resonance breaks down [17].…”

Short-Pulse Laser-Produced Plasmas

“…With lasers of ≈100-fs duration, hydrodynamic simulations (see Sect.5) show that the gradient scale length L can be very much smaller than the laser wavelength λ and that typical expansion velocities are in the 0.1-nm/fs range. Absorption occurs within a layer thickness of a skin depth (≈10-nm) [23][24][25][26]. At normal incidence in the Fresnel limit (λ ≪ L) and a metallic target, laser absorption is quite small and the coupling of laser energy to target electrons is very inefficient (see Fig.1).…”